The present invention relates generally to the production of glare-reducing windows and the like and, more particularly, to an improved apparatus for feeding refrigerated plastic sheet material onto sheet supporting frames employed to shape the sheet material for use as plastic interlayers in bent, laminated safety glass windshields.
Glare-reducing laminated windshields employed in present-day automobiles generally utilize a plastic interlayer having a colored or neutral shaded band extending across the upper marginal edge portion thereof. This band is preferably graduated with the greatest concentration of light-absorbing dye being present nearest the periphery of the plastic interlayer and the concentration thereof diminishing gradually downwardly towards the other edge of the band until finally becoming almost imperceptible at the fade-off line. It has been found that this band greatly reduces discomfort to the driver and other occupants of the vehicle which normally results from direct sun glare through the windshield.
Since the windshield in present automobile designs is mounted in a tilted or non-vertical attitude and because of the longitudinal curvature of the windshield, the otherwise flat plastic interlayer having a rectilinear colored band must be shaped or warped prior to lamination to obtain a fade-off line which is horizontal and substantially parallel to the horizon when the finished curved windshield is installed in its functional inclined position in the automobile.
In a conventional plastic interlayer shaping operation, a plurality of continuous lengths of suitable thermoplastic material are withdrawn from separate supply rolls and fed by a pair of opposed, traction rollers onto an assembly table. The superimposed sheets are then clamped at their free ends in a frame and uniformly stretched or placed in tension by clamping the opposed rollers against the superimposed sheets and bodily moving such rollers in a direction opposite to the direction of sheet advancement. While maintained in this stretched condition, the continuous lengths of plastic also are clamped in the frame along a line remote from the free ends thereof and then cut transversely from the continuous lengths outwardly of the frame to provide a plurality of individual sheets. Thus, the sheets are clamped in a stretched condition in the frame along the two opposite transverse edges only. A succession of these frames, each with a plurality of uniformly taut or tensioned sheets clamped therein, are supported in a vertical orientation and moved in spaced relation first through a heating zone and then through a cooling zone. During heating, the sheets become pliable and the central portions, as well as the unsupported longitudinal edges thereof, sag downwardly by gravity in the plane of the sheets to the desired curvatures or shapes. Sometimes, external tractive forces are applied to the lower, unsupported edges of the sheets to assist in distorting or warping the sheets to the desired curvatures. While maintained in such distorted or warped condition, the sheets are cooled to set the same in these desired shapes.
While the traction rollers utilized to feed the sheet material in the above described process admirably serve the purpose for which they are intended, they do possess certain disadvantages. For example, the tractive forces are concentrated along a narrow line of contact between the rollers and the sheet material, tending to crimp or distort the sheet material. This is especially so upon clamping the sheet material therebetween during the sheet stretching operation. Such deformed portions must be cut from the continuous stock and discarded, resulting in wastage of the expensive thermoplastic material.
Another problem in using the conventional thermoplastic materials in their natural state is that a parting agent, in the form of a fine powdery substance, is employed between adjacent convolutions of the supply rolls to prevent adhesion therebetween and to facilitate the dispensing and handling thereof. However, the parting agent not only creates a dusty and polluted environment but, more importantly, requires costly removal as by washing prior to the final assembly of the plastic sheets with the glass panels. Consequently, refrigerated thermoplastic material has been developed to obviate the need for a parting material and the shortcomings associated therewith. However, due to the rigidity of such refrigerated plastic material, the pressure of the opposed rollers applied against the sheets to feed the same tend to crack, rupture and/or somewhat distort the sheet material, thereby destroying its utility as interlayers in the laminated glass, and particularly in windshields, where optical requirements must be maintained within close tolerances.